Description

I think this is a bug. Or at the very least, it can make for unexpected behaviour that is difficult to trace. When developing locally, DEBUG and TEMPLATE_DEBUG are often True. When TEMPLATE_DEBUG is True, compiling a template that contains an {% include %} to a template that doesn't exist will raise TemplateDoesNotExist at compile time of the parent template.

The problem is that when you try to conditionally load a template, and perform a different action if it does not exist. For example, you might have a view that uses select_template to load one of several templates (if one exists) and send an email. If no email was sent (because no template exists), render a different template/context to the response or take some other action. If the email template contains a typo in the name of an included template, the email template will not be loaded and no email will be sent because TemplateDoesNotExist was raised.

In the local dev environment, this will not display a pretty debug page showing the error inside the email template, it will simply behave unexpectedly (by not sending an email when the email template DOES exist). On the production server (or when TEMPLATE_DEBUG is False), it will behave as expected. The email template will be found and it will be rendered.

This problem doesn't exist when using a variable as the template name.

>>> t = Template('{% include somevar %}')
>>>

Why does Django try to execute the {% include %} when it contains a hard coded string at compile time at all? I think this is a case of premature optimisation. If it execution of the {% include %} was delayed until render time, select_template would correctly return the email template when TEMPLATE_DEBUG is True, and the proper error would be displayed in the rendered email template (making it an easy fix to correct the typo).

Technically, the problem is the implementation of django.template.loader_tags.ConstantIncludeNode: the template resolution with get_template should be moved from __init__ to render.
I'm attaching a patch that fixes it with the minimum amount of changes, just to show the idea. However:

after the patch, ConstantIncludeNode and IncludeNode are very similar; some refactoring would be useful,

Summary
changed from {% include %} tag raises TemplateDoesNotExist at compile time of parent template if TEMPLATE_DEBUG is True to {% include %} tag raises TemplateDoesNotExist at compile time if TEMPLATE_DEBUG is True